Prenylation of proteins by intermediates of the isoprenoid biosynthetic pathway represents a new class of post-translational modification (Glomset, J. A., Gelb, M. H., and Famsworth, C. C. (1990). Trends Biochem. Sci. 15, 139-142; Maltese, W. A. (1990). FASEB J. 4, 3319-3328). This modification typically is required for the membrane localization and function of these proteins. Prenylated proteins share characteristic C-terminal sequences including CaaX (C, Cys; a, usually aliphatic amino acid; X, another amino acid), XXCC, or XCXC. Three post-translational processing steps have been described for proteins having a C-terminal CaaX sequence: addition of either a 15 carbon (farnesyl) or 20 carbon (geranytgeranyl) isoprenoid to the Cys residue, proteolytic cleavage of the last 3 amino acids, and methylation of the new C-terminal carboxylate (Cox, A. D. and Der, C. J. (1992a). Critical Rev. Oncogenesis 3:365-400; Newman, C. M. H. and Magee, A. I. (1993). Biochim. Biophys. Acta 1155:79-96). Some proteins may also have a fourth modification: palmitoylation of one or two Cys residues N-terminal to the farnesylated Cys. Proteins terminating with a XXCC or XCXC motif are modified by geranylgeranylation on the Cys residues and do not require an endoproteolytic processing step. While some mammalian cell proteins terminating in XCXC are carboxymethylated, it is not clear whether carboxymethylation follows prenylation of proteins terminating with a XXCC motif (Clarke, S. (1992). Annu. Rev. Biochem. 61,355-386). For all of the prenylated proteins, addition of the isoprenoid is the first step and is required for the subsequent steps (Cox, A. D. and Der, C. J. (1992a). Critical Rev. Oncogenesis 3:365-400; Cox, A.D. and Der, C. J. (1992b) Current Opinion Cell Biol. 4:1008-1016).
Three enzymes have been described that catalyze protein prenylation: farnesyl-protein transferase (FPTase), geranylgeranyl-protein transferase type I (GGPTase-I), and geranylgeranyl-protein transferase type-II (GGPTase-II, also called Rab GGPTase). These enzymes are found in both yeast and mammalian cells (Clarke, 1992; Schafer, W. R. and Rine, J. (1992) Annu. Rev. Genet. 30:209-237). FPTase and GGPTase-I are .alpha./.beta. heterodimeric enzymes that share a common .alpha. subunit; the .beta. subunits are distinct but share approximately 30% amino acid similarity (Brown, M. S. and Goldstein, J. L. (1993). Nature 366, 14-15; Zhang, F. L., Diehl, R. E., Kohl, N. E., Gibbs, J. B., Giros, B., Casey, P. J., and Omer, C. A. (1994). J. Biol. Chem. 269, 3175-3180). GGPTase. II has different .alpha. and .beta. subunits and complexes with a third component (REP, Rab Escort Protein) that presents the protein substrate to the .alpha./.beta. catalytic subunits. Each of these enzymes selectively uses farnesyl diphosphate or geranylgeranyl diphosphate as the isoprenoid donor and selectively recognizes the protein substrate. FPTase farnesylates CaaX-containing proteins that end with Ser, Met, Cys, Gln or Ala. GGPTase-I geranylgeranylates CaaX-containing proteins that end with Leu or Phe. For FPTase and GGPTase-I, CaaX tetrapeptides comprise the minimum region required for interaction of the protein substrate with the enzyme. GGPTase-II modifies XXCC and XCXC proteins; the interaction between GGPTase-II and its protein substrates is more complex, requiting protein sequences in addition to the C-terminal amino acids for recognition. The enzymological characterization of these three enzymes has demonstrated that it is possible to selectively inhibit one with little inhibitory effect on the others (Moores, S. L., Schaber, M. D., Mosser, S. D., Rands, E., O'Hara, M. B., Garsky, V. M., Marshall, M. S., Pompliano, D. L., and Gibbs, J. B., J. Biol. Chem., 266:17438 (1991)).
The characterization of protein prenylation biology and enzymology has opened new areas for the development of inhibitors which can modify physiological processes. The prenylation reactions have been shown genetically to be essential for the function of a variety of proteins (Clarke, 1992; Cox and Der, 1992a; Gibbs, J. B. (1991). Cell 65: 1-4; Newman and Magee, 1993; Schafer and Rine, 1992). This requirement often is demonstrated by mutating the CaaX Cys acceptors so that the proteins can no longer be prenylated. The resulting proteins are devoid of their central biological activity. These studies provide a genetic "proof of principle" indicating that inhibitors of prenylation can alter the physiological responses regulated by prenylated proteins.
Protein geranylgeranyltransferase type-I (GGTase-I) transfers a geranylgeranyl group from the prenyl donor geranylgeranyl diphosphate to the cysteine residue of substrate proteins containing a C-terminal CAAX-motif in which the "X" residue is leucine or phenylalanine (Clark, 1992; Newman and Magee, 1993). Known targets of GGTase-I include the gamma subunits of brain heterotrimeric G proteins and Ras-related small GTP-binding proteins such as RhoA, RhoB, RhoC, CDC42Hs, Racl, Rac2, RaplA and RaplB (Newman and Magee, 1993; Cox and Der, 1992a). The proteins RhoA, RhoB, RhoC, Rac1, Rac2 and CDC42Hs have roles in the regulation of cell shape (Ridley, A. J. and Hall, A. (1992). Cell 70:389-399; Ridley, A. J., Paterson, H. F., Johnston, C. L., Keikmann, D., and Hall, A. (1992). Cell 70:401-410; Bokoch, G. M. and Der, C. J. (1993). FASEB J. 7:750-759). Rac and Rap proteins have roles in neutrophil activation (Bokoch and Der, 1993 ).
Activation of growth factor function and Ras function can cause rumor formation. Recently, it was demonstrated that the Rho and Rac proteins transmit intracellular signals initiated by growth factors and by Ras protein (Prendergast, G. C. and Gibbs, J. B. (1993). Adv. Cancer Res. 62, 19-64; Ridley and Hall, 1992; Ridley et al., 1992). Specifically, experiments demonstrated that the function of Rho and Rac proteins was required by Ras and growth factors to change cell shape, a biological parameter indicative of cellular transformation and cancer. Since Rho and Rac proteins require geranylgeranylation for function, an inhibitor of GGPTase-I would block the functions of these proteins and be useful as an anticancer agent.
Neutrophil activation is part of the body's inflammatory response. (Haslett, C. et al. Cur. Opinion Immunology, 2:10-18 (1989) Geranylgeranylated Rac and Rap proteins are required for this effect (Bokoch and Der (1993); Abo, A. et al., Nature, 353:668-670 (1991); Knaus, U.G. et al. Science, 254:1512-1515 (1991); Eklund, E. A. et al. J. Biol. Chem. 266:13964-13970 (I991); Quinn, M.T. et al. Nature, 342:198-200 (1989)), so an inhibitor of GGPTase-I will have antiinflammatory activity.